Seismo Blog

Rumbles on the Red Planet

April 25, 2019

Mars, the Red Planet, is alive. No, we are not talking about a sudden success in the elusive search
for "Life on Mars". We are talking about the living interior of our immediate neighbor in the solar
system. Compared to Earth, however, the Martian signs of such tectonic life are rather weak.
Nevertheless, during the course of the past six weeks something in the innards of Mars rumbled
loudly enough, that the most sensitive seismometer ever built took note of it. The seismic sensor,
which was deployed on the planet by the Insight spacecraft late last year,
recorded such very slight vibrations on at least four occasions.

In a press release, Nasa's Jet Propulsion Laboratory in Pasadena has already celebrated, that Insight has now recorded "likely marsquakes". They even turned the recording into an audio file. But many seismologists currently participating in the annual meeting of the Seismological Society of America in Seattle, are not so sure that they were really quakes. The reason for their skepticism: the seismograms from Mars do not look at all like the recordings of a typical terrestrial temblor.

Earthquakes are caused – as often explained in this blog – by a sudden rupture of the bond between two flanks of an earthquake fault. The main emphasis here is on the word "sudden". The rapid shift of huge rock masses generates several kinds of seismic waves, which travel at different characteristic speeds through or around the Earth. When the waves are recorded by a seismometer, they leave traces which are dominated by clear and distinct pulses, called phases or onsets. Figure 1 shows a typical such recording from an earthquake in Indonesia, captured by one of the seismometers of the Berkeley Digital Seismic Network near the town of Columbia in the foothills of the Sierra Nevada. The different pulses in the seismogram are marked by arrows.

In contrast, the recordings from Mars do not show any such clear pulses. Instead the "marsogram" looks much more like the record of a deep hum, which gently increases in amplitude over the course of five minutes, only to slowly decay over a similar time interval (see Figure 2). The event shown here took place on April 7, the other three rumblings captured by Insight were much weaker.

To us terrestrial seismologists, the lack of clear "phases" in the recording is an indication that these ground vibrations on Mars were not caused by the sudden shifts of the flanks of a marsquake fault. In his keynote talk at the conference in Seattle, Bruce Barnerdt, the chief scientist of the Insight mission, likened them to certain types of quakes on the Moon. Others speculated that these rumblings may be caused by a process also known from some of the Earth's tectonically active zones. In these so-called "Slow Slip Events" the ground does not shift suddenly and rapidly with great force. Instead the two flanks rub against each other over the course of hours or days. The resulting rumblings are of an intensity much too low to be felt by humans. However seismometers record them as "seismic tremor". The seismograms of these tremors look a lot the one now transmitted from the surface of Mars. So maybe, Insight hasn't discovered the first marsquakes but has sent data from the first recordings of a "Martian tremor". Barnerdt and his colleagues are optimistic, however, that their several hundred million dollar seismometer will eventually record vibrations caused by a real marsquake during its planned two year lifetime. (hra165)